1. Field of Application
The present invention relates to an electric generator apparatus of a vehicle, and in particular to an electrical generator apparatus which applies control for achieving electrical power generation with reduced fuel consumption.
2. Description of Related Art
In recent years, due to the increasing price of fuel for motor vehicles, there is an increased need for operating vehicles with minimized fuel consumption. The assignees of the present invention have previously proposed (in Japanese Patent First Publication No. 2004-260908, referred to in the following as reference document 1), in regard to reducing fuel costs, a method whereby electric power costs are calculated respectively separately for each of a plurality of electric power sources. For each power source, the corresponding electric power cost is calculated as the amount of fuel (e.g., measured in grams) consumed in generating a unit amount (e.g., one kWh) of electrical energy by that electric power source.
With that method, priority is assigned to supplying power that is produced from the electric power source which provides the lowest-cost energy. In the following, such a method of controlling an electrical generator apparatus will be referred to as power cost reduction generation control.
By using such control, when for example an excess of power is produced from an electric generator which has a low electric power cost (i.e., power is being generated in excess of the power required by the electrical loads supplied by that generator) the excess power can be stored in an electrical energy storage device. For the purpose of description in the following, it will be assumed that each electrical energy storage device is a storage battery, although other types of device such as an electric dual-layer capacitor can be used in such applications. Subsequently, when power is being produced from the electric generator at a relatively high generation cost, changeover can be performed to supplying power to the electrical loads by discharging power from the battery, i.e., the battery is utilized as an electric power source.
In that way, by appropriately controlling the electric power sources in accordance with the costs (measured in terms of consumed fuel) of generating electrical power, reduced fuel consumption can be achieved
Furthermore, in recent years (as described for example in Japanese Patent First Publication No. 2001-309574, referred to in the following as reference document 2) a dual-voltage type of vehicle-use electrical power supply apparatus has been proposed. This has a high-voltage power supply system having a high-voltage electric generator and high-voltage battery, for supplying power to electrical loads that operate at a high voltage, and a low-voltage power supply system and low-voltage battery, for supplying power to electrical loads requiring a low voltage. A DC-DC converter is connected between the high-voltage power supply system and low-voltage power supply system, and is controlled for transferring power between them. Such a dual-voltage electric generator apparatus can reduce fuel costs, since a high-voltage system can operate more efficiently (i.e., with lower electrical losses) than a low-voltage system.
With the power cost reduction generation control method previously proposed by the assignees of the present invention, considering a single power supply system (made up of an electric generator that supplies power to electrical loads and to a battery), if variation of the supply voltage of the system must be limited to within a narrow range then it is necessary to utilize a high-capacity battery in order to achieve the desired reduction in fuel costs while sufficiently limiting the amount of supply voltage variation. In addition, the battery must be capable of withstanding deterioration caused by a high frequency of charge/discharge cycles. The cost of a suitable type of battery (e.g., a lithium-ion secondary battery), measured as the cost per unit of charge storage capacity, is substantially higher than the conventional type of lead-acid battery used in a vehicle. For these reasons, the cost of such a battery will be high, and it will occupy a large amount of space. Due to such disadvantages, it has been difficult to put the power cost reduction generation control method into practical application.
The above problems will be described more specifically in the following.
The basis of power cost reduction generation control is to attempt to generate electrical power at the lowest possible generation cost, irrespective of the level of power being consumed in the electrical loads. To achieve this, at each point in time, the difference between the amount of electrical power being generated at that time and the amount of power being consumed by the electrical loads is either (when in excess) stored in a battery, or (when in deficit) is supplied to the electrical loads by discharge from the battery.
For example, the average per-unit cost of the electrical energy that is currently held stored in the battery can be compared with the per-unit cost of electrical energy that is currently being produced by the generator, and if the energy is being produced by the generator at lower cost then that stored in the battery, then the level of output power of the generator can be increased, to thereby increase the amount of charge held in the battery. In that way, the average per-unit cost of the energy that is stored in the battery will be lowered. On the other hand, if electrical energy is being produced by the generator at higher per-unit cost then that stored in the battery, then discharging of the battery can be performed, to use the battery as a power source, while the level of output power of the generator is reduced.
The problem with respect to battery size and cost can be readily understood as follows. To achieve an effective reduction of fuel consumption with such a power cost reduction generation control method, if the amount of variation of the supply voltage must be limited to a narrow range, it is necessary for the maximum amount of energy that can be stored in the battery to be substantially greater than for a conventional type of battery used in a motor vehicle. This is due to the fact the output terminal of the electric generator is connected to the battery terminal of the corresponding battery and to the electrical loads, with the loads thus being supplied at a voltage that is substantially equal to the terminal voltage of the battery. However the terminal voltage of a battery varies substantially in accordance with the level of charge in the battery.
Thus, in order to sufficiently limit the amount of variation of the supply voltage of the electrical loads, it is necessary to limit the degree of variation of the level of charge stored in the battery. However as described above, with the power cost reduction generation control method, a battery is used as a power source at certain times, so that it is necessary that a large amount of power can be discharged from the battery.
To achieve this, while limiting the amount of variation of the terminal voltage of the battery (and hence the amount of variation of the supply voltage of the corresponding loads) to a narrow range, it is necessary for the battery to have a large charge storage capacity, and hence to be large in scale. Such a battery will be heavy and will occupy a large amount of space, and will be high in cost.
In addition to that disadvantage, application of power cost reduction generation control will result in a higher frequency of performing charging/recharging cycles for the battery. With a conventional type of lead-acid battery this will result in an excessively shortened operating lifetime. Hence there is the further problem that not only must each battery be large in capacity, but each battery must be implemented as a device (such as a lithium-ion secondary battery) that has a sufficiently long operating life in spite of a high frequency of charging/recharging cycles.
Since such an alternative type of electric charge storage device is substantially more expensive than a conventional lead-acid battery, then if it is also necessary for that device to have large charge storage capacity and so be made large in scale (due to applying power cost reduction generation control while also limiting the variation of the supply voltage to a narrow range as described above), then the cost of the electric charge storage device will become excessive.
These problems are significant obstacles to achieving practical application of power cost reduction generation control.